Metabolic control analysis of insulin-stimulated glucose disposal in rat skeletal muscle

Metabolic control analysis was used to calculate the distributed control of insulin-stimulated skeletal muscle glucose disposal in awake rats. Three separate hyperinsulinemic infusion protocols were performed: 1) protocol I was a euglycemic (approximately 6 mM)-hyperinsulinemic (10 mU. kg(-1). min(-1)) clamp, 2) protocol II was a hyperglycemic ( approximately 11 mM)-hyperinsulinemic (10 mU. kg(-1). min(-1)) clamp, and 3) protocol III was a euglycemic (approximately 6 mM)-hyperinsulinemic (10 mU. kg(-1). min(-1))-lipid/heparin (increased plasma free fatty acid) clamp. [1-13C]glucose was administered in all three protocols for a 3-h period, during which time [1-13C]glucose label incorporation into [1-13)]glycogen, [3-13C]lactate, and [3-13C]alanine was detected in the hindlimb of awake rats via 13C-NMR. Combined steady-state and kinetic data were used to calculate rates of glycogen synthesis and glycolysis. Additionally, glucose 6-phosphate (G-6-P) was measured in the hindlimb muscles with the use of in vivo 31P-NMR during the three infusion protocols. The clamped glucose infusion rates were 31.6 +/- 2.9, 49.7 +/- 1.0, and 24.0 +/- 1.5 mg. kg(-1). min(-1) at 120 min in protocols I-III, respectively. Rates of glycolysis were 62.1 +/- 10.3, 71.6 +/- 11.8, and 19.5 +/- 3.6 nmol. g(-1). min(-1) and rates of glycogen synthesis were 125 +/- 15, 224 +/- 23, and 104 +/- 17 nmol. g(-1). min(-1) in protocols I-III, respectively. Insulin-stimulated G-6-P concentrations were 217 +/- 8, 265 +/- 12, and 251 +/- 9 nmol/g in protocols I-III, respectively. A top-down approach to metabolic control analysis was used to calculate the distributed control among glucose transport/phosphorylation [GLUT-4/hexokinase (HK)], glycogen synthesis, and glycolysis from the metabolic flux and G-6-P data. The calculated values for the control coefficients (C) of these three metabolic steps (C(J)(GLUT-4/HK) = 0.55 +/- 0.10, C(J)(glycogen syn) = 0.30 +/- 0.06, and C(J)(glycolysis) = 0.15 +/- 0.02; where J is glucose disposal flux, and glycogen syn is glycogen synthesis) indicate that there is shared control of glucose disposal and that glucose transport/phosphorylation is responsible for the majority of control of insulin-stimulated glucose disposal in skeletal muscle.